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  1. #1

    65U Rebuild Question - Cause of death?

    I'm currently rebuilding a 65U. I'm trying to find the cause of death. The last cylinder had really low compression. When I took the jugs off the piston seemed relatively fine and the cylinder wasn't really destroyed or anything but the rings were like inside the grooves there was no way for them to seal the cylinder. What causes this? A lean condition? I don't have any pictures unfortunately the original tear down was last August and I'm just getting around to finishing this thing up now.


    I have not down a leak down test but will after I assemble possibly before. Could I do a quick leak down test with no pistons in and just the jugs and head on to check to see if the crank seal is bad?


  2. #2
    Tool Bag water worx's Avatar
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    The rings stick in the pistons usually from excess carbon.

    No you cant do a leakdown unless the motor is completely assembled.

    Just replace the crank seals if you already have it half way apart.

  3. #3
    Why can't I do a leakdown if I put the juggs and head back on just temporarily to see If it will hold pressure? Problem is the motor is in a jetboat and I don't need to remove the motor unless I need to replace crank seal or bottom end stuff. So if it passes the leakdown I'm just gonna put it back together in the boat with new pistons and cylinder and call it a day.

    The rings were completely flush with the sides of the piston which is why it wasn't making compression in that hole. Could I have just re expanded the rings and clean out the ring-lands and been alright?

  4. #4
    Tool Bag water worx's Avatar
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    Let me ask you this ....have you ever slipped a Yamaha 3 cyl jug on while trying to not break a ring while in the boat ??

    If you have then your a better mech than me & I have been doing it a long time.

    Yes on that last question.

  5. #5
    I've done it before in my boat this is a ls2000 jetboat not a jet ski theres enough room to stand in the engine compartment. I'm new to two strokes but I've rebuilt many motors, mostly old school chevies and industrial diesel motors, and I'm a semi professional mechanic (construction equipment).


    That drives me nuts!! I thought that maybe it leaned out got hot and pushed the rings into the piston and thats why they were like that. I remember considering just putting it back together but I figured do it right and got a cylinder exchange.

  6. #6
    Tool Bag water worx's Avatar
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    Good luck

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  8. #7
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    you might want to try to find a thread where a fellow was about to launch like a MAV as he couldn't get the jugs back on due to ring issues and the motor if memory serves was sitting on the bench.

    I for one would not want to even try that. I had a hard enough time dealing with other stuff inside the boat.

    I've stopped working on them for that reason ( well I still do jetpumps)

  9. #8
    ncdoo's Avatar
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    He did say to reassemble without the pistons so that would work. As far as unsticking the rings, I've never has any luck with that. End up breaking the rings or damaging the piston.

  10. #9
    I've disassembled the other motor in this boat before its a pain but I ended up putting the piston in the first cylinder before (without wrist pin in) hand and leaving the wrist pin hole below the jug base then getting the crankshaft in a way so that the first connecting rod was down relative to the other throws and getting the second and third pistons in with the wrist pins connected then lowering the jug down and inserting the wrist pin for the first and finally very carefully installing the outer circlip. Took me 15 minutes after I tried it that way before I tried that I fought with it for hours.

  11. #10
    Tool Bag water worx's Avatar
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    Much easier/faster to remove motor put on bench reassemble,reinstall.

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